Human impact on vegetation at the Alpine tree-line ecotone during the last millennium: lessons from high temporal and palynological resolution

Three mires and a small lake in the Swiss and Austrian Alps were studied palynologically at high resolution, covering the last 1,000, 400, 50 and 1,200 years, respectively. Methodological lessons include: (1) Sub-decadal resolution in upper, little-decomposed peat layers reveals recurrent marked fluctuations in both percentages and influx of regional tree-pollen types, reflecting variations in pollen production rather than in plant-population sizes. (2) Intermittent, single-spectrum pollen maxima in samples of sub-decadal resolution indicate pollen transport in clumps. This type of pollen transport may remain unrecognized in sections with lower sampling resolution, which may then lead to inappropriate interpretation in terms of plant-population sizes. (3) The detection of short-lived phases of human impact in decomposed peat requires sampling intervals as close as 0.2 cm. (4) PAR (pollen influx) may reflect vegetation dynamics more faithfully than percentages. Reliable PAR, however, is difficult to achieve in Alpine mires due to past human impact on peat growth, even when complex depth–age modelling techniques are used. Critical comparison of PAR with percentages is therefore essential. (5) Careful consideration of spatial scales in pollen signals (local–regional and subdivisions) is essential for a realistic palaeo-ecological interpretation. Results in terms of past human impact on vegetation are summarized as follows: (1) Trends in pollen types reflecting regional human action are in general agreement with earlier findings for the western Swiss Alps, allowing for regional differences. (2) All mires in the Alps investigated here and in an earlier study experienced human impact during the last millennium. The studied small lake, lying in sub-alpine pasture, records forest dynamics at a lower elevation since a.d. 800.     

Decomposition Patterns of Foliar Litter and Deadwood in Managed and Unmanaged Stands: A 13-Year Experiment in Boreal Mixedwoods

Litter decomposition is a major driver of carbon (C) and nitrogen (N) cycles in forest ecosystems and has major implications for C sequestration and nutrient availability. However, empirical information regarding long-term decomposition rates of foliage and wood remains rare. In this study, we assessed long-term C and N dynamics (12–13 years) during decomposition of foliage and wood for three boreal tree species, under a range of harvesting intensities and slash treatments. We used model selection based on the second-order Akaike’s Information Criterion to determine which decomposition model had the most support. The double-exponential model provided a good fit to C mass loss for foliage of trembling aspen, white spruce, and balsam fir, as well as aspen wood. These litters underwent a rapid initial phase of leaching and mineralisation, followed by a slow decomposition. In contrast, for spruce and fir wood, the single-exponential model had the most support. The long-term average decay rate of wood was faster than that of foliage for aspen, but not of conifers. However, we found no evidence that fir and spruce wood decomposed at slower rates than the recalcitrant fraction of their foliage. The critical C:N ratios, at which net N mineralisation began, were higher for wood than for foliage. Long-term decay rates following clear-cutting were either similar or faster than those observed in control stands, depending on litter material, tree species, and slash treatment. The critical C:N ratios were reached later and decreased for all conifer litters following stem-only clear-cutting, indicating increased N retention in harvested sites with high slash loads. Partial harvesting had weak effects on C and N dynamics of decaying litters. A comprehensive understanding of the long-term patterns and controls of C and N dynamics following forest disturbance would improve our ability to forecast the implications of forest harvesting for C sequestration and nutrient availability.     

Relationship between deterrence and toxicity of plant secondary compounds for the alfalfa weevil Hypera brunneipennis

A variety of plant secondary compounds, several of which are quite widespread in nature were tested for their deterrence to the specialist coleopteran Hypera brunneipennis (Boheman) in short-term behavioral assays. The compounds were nicotine, quinine, sparteine, hordenine, linamarin, amygdalin, sinigrin, morin, juglone, chlorogenic acid, digitonin, mimosine, diosgenin, rutin and ursolic acid. Nine of these were then tested for their post-ingestional effects over one to two weeks of adult life, using fecundity as a measure of the effects. In only one case was there any indication of a detrimental effect or any trend suggesting one. The evolutionary implications of these findings are discussed.